Clarify middleware.ts location in docs

[stuartkeith]

I had an issue where middleware stopped running. It turns out that because I had moved the pages directory from /pages to /src/pages, I also needed to move /middleware.ts to /src/middleware.ts.

While double checking the docs to figure out this problem, I interpreted the docs as saying middleware.ts could go in either the root or src. I thought swapping the sentence around would more clearly indicate middleware.ts must be alongside pages, wherever that is.

[ijjk]

Inverting this for clarity seems alright, thanks for the PR!